With a view to obviate the need for wired cabling connections between stations in local area networks (LANs), wireless local area networks have been developed, and are now commercially available. These wireless local area networks employ a plurality of mobile network stations, which may be data processing devices (such as personal computers) having wireless communication capability.
In wired based networks, collision detection can be accomplished with a relative ease. However, for wireless based networks, which use a single channel, it is substantially difficult to detect collisions due to the large dynamic range of receive signal levels. Therefore, wireless local area networks typically employ a collision avoidance scheme, instead of collision detection.
Wireless local area networks, LANs, are generally configured based on a medium access control (MAC) arrangement that employs a listen-before-talk scheme like CSMA/CA (carrier sense multiple access with collision avoidance) as described by the IEEE 802.11 standard. In accordance with one embodiment described in the IEEE 802.11 standard, a wireless local area network includes an access point, which serves as a base station and a plurality of other network stations. The network stations within a group or a cell communicate directly to their corresponding access point. This access point forwards messages to the destination station within the same cell or through a wired distribution system to another access point, from which such messages arrive finally at a desired destination station.
In accordance with the medium access control (MAC) arrangement, each local area network station begins transmission when it determines that no other station is transmitting communication signals. To this end, each station defers its transmission of signals, so long as the signal level it receives from other stations is above a specifiable receive threshold level. Thus, the medium access control (MAC) arrangement prevents a second station remotely located from the first station, to start signal transmission that overlaps in time with an earlier started transmission by the first station. Typically, the second station defers its signal transmission for a randomly selected period of time.
A very short duration carrier detect turnaround time is fundamental for this random wait characteristic. For example, IEEE 802.11 DSSS (direct sequence spread spectrum) standard calls for a slotted random wait behavior based on 20 .mu.sec (microsecond) time slots, which cover the carrier detect turnaround time.
Additionally, the medium access control (MAC) described by the IEEE 802.11 standard calls for one signal threshold level for both receive and defer modes. The lowest level of the receive threshold is also the level used for deferring. Accordingly, if the receiver detects any other signal above the receive threshold, it will stop transmitting. The IEEE 802.11 DSSS standard specifies a defer threshold that has to be equal or more sensitive than -70 dBm at a transmit power below 50 mW, -76 dBm at a transmit power between 50 and 100 mW, and -80 dBm at a transmit power between 100 mW and 1 Watt.
Referring to FIG. 1, the prior art method of providing a collision avoidance scheme is shown and described. Specifically, the IEEE 802.11 CSMA/CA protocol is designed to reduce the collision probability between multiple stations accessing the medium at the point where they would most likely occur. The highest probability of a collision would occur at the point in time just after the medium becomes free, following a busy medium. This is because multiple stations would have been waiting for the medium to become available again. Therefore, a random back off arrangement is used to resolve medium contention conflicts. As is evident to those skilled in the art, a very short duration carrier detect turnaround time is fundamental for this random wait characteristic. In addition, the IEEE 802.11 medium access control, MAC, defines an option for medium reservation via RTS/CTS (request-to-sender/clear-to-send) polling interaction and point coordination for time bounded services. As shown in FIG. 1, after a busy medium period, all wireless LAN devices have to wait during a so called IFS (inter frame spacing) period and then they can attempt to transmit after waiting the required random number of slot time intervals as long as there are no other transmissions.
With reference to co-channel medium reuse, the IEEE 802.11 DSSS standard provides for acceptable capture properties, which allows the usage of a less sensitive defer threshold leading to better medium reuse conditions. However, a less sensitive defer threshold means that the range at which the corruption of transmissions is prevented is smaller. In access point based networks and ad-hoc networks with a server station, the traffic goes to and from the access point or server station. The minimum receive level at which access point/server station and its assigned stations receive each other is difficult to predict due to the multipath fading and shadowing effects and due to the changing distance of a mobile network station from the access point.
Thus, there is a need for an improved medium access arrangement that allows for successful reception at a lower level and yet provides for a substantially high co-channel medium reuse and substantially low power consumption.